In chemical vapor deposition (CVD), a solid component is deposited from the gas phase on a heated substrate surface due to a chemical reaction. A special feature of this method is that it allows deposition of conformal films. In contrast to classical physical vapor deposition (PVD), chemical vapor deposition also allows coating of complex three-dimensionally shaped surfaces. This allows coating of, for example, the inner surface of hollow bodies or very fine grooves in wafers. A prerequisite for deposition from the gas phase is the existence of volatile precursor compounds of the film material, from which the solid film is deposited at a specific reaction temperature. In contrast to competing gas phase reactions, chemical vapor deposition processes are usually performed at reduced pressure (typically at 0.01-10 hPa) to promote the desired reactions on the surface and thus prevent the formation of solid particles in the gas phase, and to transfer more material into the gas phase when using less volatile precursors.
N-type semiconductive metal sulfide thin films are preferably used as buffer layers between the window layer and the absorber layer in solar cells, thereby allowing a significant increase in efficiency. A “buffer layer” is understood to be a layer having a higher band gap than the adjacent semiconducting absorber layer. This higher band gap can be achieved by alloying or by suitable material selection. Recombination in the interface area of the pn junction is reduced by improved interface conditions, leading to an increase in the open terminal voltage. The buffer layer is also intended to optimize the band alignment. CdS is an n-type semiconductor having a band gap of 2.4 eV and thus absorbs in the UV and blue regions of the solar spectrum. The thereby generated electron-hole pairs are not separated by the space charge region and, therefore, do not contribute to the current.
Increasingly, efforts are being made to substitute the toxic CdS layer by less toxic materials. In this connection, various variants of the CVD method are increasingly used, which also allows for good coverage of rough substrates. In particular, indium(III) sulfide (In2S3), which has an indirect band gap of 2 eV to 2.2 eV and, being an indirect semiconductor, therefore absorbs less light than the direct semiconductor CdS, is a promising candidate to replace the toxic CdS. Therefore, various methods for producing it have been described in the art. However, to date, it is not known to use a CVD method for producing it, since, according to the knowledge of those skilled in the art, the materials to be used tend to form solid reaction products already in the gas phase, resulting in inhomogeneous films with poor coverage on the substrate.
Methods of producing In2S3 films include direct evaporation, powder phase in a reducing atmosphere and annealing at 500° C. to 800° C., chemical bath deposition (CBD), physical vapor deposition (PVD), atomic layer vapor deposition (ALCVD) or atomic layer epitaxy (ALE), chemical spray pyrolysis (CSP), ion layer gas reaction (ILGAR), and by aerosol assisted metal-organic chemical vapor deposition (AAMOCVD). However, it is not known to produce In2S3 films directly using CVD.
German Patent DE 198 31 214 C2 describes the manufacture of various metal sulfide layers suitable for use in solar cells, based on an ion-exchange reaction. The method described therein is the two-stage ILGAR method, in which, initially, a metal salt or a metal compound, as the starting material, is sequentially deposited, preferably by dipping or spraying, on a substrate heated to a temperature above 100° C., and is subsequently reacted with a reactive gas. A two-stage spray ILGAR process for manufacturing In2S3 films for use as buffer layers in solar cells is described in N. A. Allsop et al. “Indium Sulfide Thin Films Deposited by the Spray Ion Layer Gas Reaction Technique”, Thin Solid Films 513 (2006) 52-56. In this process, initially, an InCl3/ethanol solution is sprayed onto the heated substrate, resulting in the formation of a solid In(CI,OH,O) precursor layer thereon, which is subsequently converted into an indium sulfide layer by exposure to H2S gas. The layer thickness can be controlled by repeating the two-stage cycle. In the experimental setup depicted in FIG. 1 of N. A. Allsop et al., supra, the shut-off valve required for the sequential spray ILGAR method with solid phase reaction on the substrate is shown in the H2S supply line. Contact of the sprayed InCl3/ethanol solution with the H2S gas within the reaction space is strictly avoided because otherwise particles formation would occur in the reaction space, which is absolutely undesired. For this reason, purging with inert nitrogen gas is carried out between each of the individual steps of the method. Also, the information given in chapter 5 of N. A. Allsop et al., supra, according to which the precursor deposition (In(Cl,OH)) in the spray ILGAR method has similarities to the CVD method in the microscopic domain does not indicate to one skilled in the art the specific procedures to be used, since the same paragraph makes reference to the advantages of the spray ILGAR method in the macroscopic domain and for the deposition of In2S3.
In N. Takahashi et al. “Growth of InN at High Temperature by Halide Vapor Epitaxy”, Jpn. J. Appl. Phys. Vol. 36 (1997) pp. L 743-L745, a special CVD method (vapor phase epitaxy) is described in which InCl3 is involved as a precursor and which is used to produce InN (not used for solar cells) on a substrate heated to 750° C., using reactive NH3 as a precursor gas. R. Diehl et al. “Vapor Growth of Three In2S3 Modifications by Iodine Transport”, J. of Cryst. Growth 28 (1975) 306-310 describes growing In2S3 using halogen-containing transport gases. In Y. Sawada et al. “Highly-Conducting Indium-Tin-Oxide Transparent Films fabricated by Spray CVD Using Ethanol Solution of Indium (IM) Chloride and Tin (II) Chloride”, Thin Solid Films 409 (2002) 46-50, a spray pyrolysis method having similarities to CVD is described which is used for producing InSnOx (as an ITO layer for solar cells) using an InCl3/ethanol solution. However, the described method uses no reactive precursor gas and no H2S, as is natural for oxides. The substrate is heated to temperatures of 300° C. to 350° C. K. Emits et al. “Characterisation of Ultrasonically Sprayed InxSy Buffer layers for Cu(In,Ga)Se2 Solar Cells” Thin Solid Films 515 (2007) 6051-6054 describes an ultrasonic spray pyrolysis method (chemical spray pyrolysis, CSP) in which an InCl3/alcohol-thiourea solution is sprayed onto a heated substrate (about 380° C.) to produce In2S3. In this process, H2S gas is generated in situ from the thiourea. This method is used for making buffer layers. However, the high temperature required for pyrolysis and the occurrence of contamination, in particular oxide contamination, are disadvantages of the spray pyrolysis method. Spray pyrolysis does not work at low temperatures. The use of atomic layer CVD (ALCVD, atomic layer epitaxy, ALE) for producing In2S3 (for the manufacture of buffer layers) is described in T. Asikainen et al. “Growth of In2S3 Thin Films by Atomic Layer Epitaxy” Appl. Surface Science 82/83 (1994) 122-125. This method uses H2S gas and InCl3 which is sequentially evaporated at 275° C. and absorbed as a monolayer on a substrate surface heated to 300° C. to 400° C. Another sequential ALCVD method for solar cells is described in N. Naghavi et al “High Efficiency Copper Indium Gallium Diselenide (CIGS) Solar Cells with Indium Sulfide Buffer Layers Deposited by Atomic Layer Chemical Vapor Deposition (ALCVD)” Prog. Photovolt: Res. Appl. 2003; 11:437-443, where indium acetylacetonate In(acac)3 evaporated at 125° C. and H2S gas are used at substrate temperatures of 160° C. to 260° C.
Afzaal et al.: “Metal-Organic Chemical Vapor Deposition of β-In2S3 Thin Films Using a Single-Source Approach” J. of Mat. Sc: Mat. in Electr. 14 (2003) 555-557 describes an aerosol-assisted metal-organic CVD method (AAMOCVD), which uses a single, sulfur-containing precursor to be manufactured separately for producing In2S3 on a substrate heated to 425° C. to 475° C. Both Barone et al. “Deposition of Tin Sulfide Thin Films from Tin(IV) Thiolate Precursors” J. Mater. Chem., 2001, 11, 464-468 and Parkin et al. “The First Single Source Deposition of Tin Sulfide Coatings on Glass: Aerosol-Assisted Chemical Vapor Deposition using [Sn(SCH2CH2S)2]” J. Mater. Chem., 2001, 11, 1486-1490 describe the use of AACVD to produce tin sulfide for photovoltaic materials. In Barone et al., the following products are obtained depending on the substrate temperature: SnS (500° C.), SnS2 (350° C. to 450° C.) and Sn2S3 (400° C.). In this method, complex precursors, such as tin phenyl sulfur, or simple precursors, such as tin chloride, are used together with H2S gas which, in the case of a sulfur-containing precursor, is primarily intended to prevent the formation of oxides, and in the case of a sulfur-free precursor is involved in the reaction.
In K. C. Molloy et al.: “New Precursors for the Chemical Vapour Deposition of Tin Sulphide Semiconductors and Related Materials” EPSRC Reference GR/L56442/01 and GR/L54721/01, 2001, atmospheric pressure chemical vapor deposition (APCVD) is used to produce an n-type semiconductive tin sulfide thin film (tin(III) sulfide Sn2S3) on a substrate heated to a temperature between 475° C. and 525° C. In this process, two precursors, namely a tin-containing soluble precursor (SnCI4 (a liquid having a boiling point of 114° C.) or SnBr4) and hydrogen sulfide (H2S) as a reactive gaseous precursor are used in an inert nitrogen carrier gas stream. Reaction of SnCI4 or SnBr4 with H2S-Gas yielded corresponding homogeneous single-phase films on large glass plates as substrates. The deposition parameters monitored during the process were substrate temperature, precursor concentration, and gas flow rate. It was found that the substrate temperature had the greatest effect on the synthesis (brown Sn2S3 was obtained at 475° C. to 525° C.), whereas variation of the flow rate of the H2S gas had no effect on the film stoichiometry. With an unlimited supply of H2S, there was a proportionality between the deposition rate and the rate at which the tin precursor was supplied. A production process in which no toxic H2S gas is used; i.e., in which sulfur is supplied via the solid precursor, can only be achieved using AACVD, because the vapor pressure (boiling point) of the sulfur-containing precursors used is too low for APCVD. It should also be noted that the reaction behavior of Sn differs from that of In (different main group). Therefore, it is not readily apparent to one skilled in the art to make In2S3 using APCVD based on simple conclusions drawn by analogy.